1. Field of the Invention
This invention relates generally to a system and method for determining whether an injector that injects hydrogen gas into the anode side of a fuel cell stack is operating properly and, more particularly, to a system and method for determining whether an injector that injects hydrogen gas into the anode side of a fuel cell stack is operating properly by spectrally analyzing a voltage response of the stack.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cell systems as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines. Fuel cell vehicles are expected to rapidly increase in popularity in the near future in the automotive marketplace.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically, but not always, include finely divided catalytic particles, usually a highly active catalyst such as platinum (Pt) that is typically supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow fields are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow fields are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The membranes within a fuel cell stack need to have sufficient water content so that the ionic resistance across the membrane is low enough to effectively conduct protons. Membrane humidification may come from the stack water by-product or external humidification. The flow of reactants through the flow channels of the stack has a drying effect on the cell membranes, most noticeably at an inlet of the reactant flow. However, the accumulation of water droplets within the flow channels could prevent reactants from flowing therethrough, and may cause the cell to fail because of low reactant gas flow, thus affecting stack stability. The accumulation of water in the reactant gas flow channels, as well as within the gas diffusion layer (GDL), is particularly troublesome at low stack output loads.
A technique known in the art for determining membrane humidification uses high frequency resistance (HFR) humidification measurements. HFR humidification measurements are ascertained by providing a high frequency component or AC signal on the electrical load of the stack so that a high frequency ripple is produced on the current output of the stack. High frequency resistance is a well-known property of fuel cells, and is closely related to the ohmic resistance, or membrane protonic resistance, of the fuel cell membrane. Ohmic resistance is itself a function of the degree of fuel cell membrane humidification. Therefore, by measuring the HFR of the fuel cell membranes of a fuel cell stack within a specific band of excitation current frequencies, the degree of humidification of the fuel cell membrane may be determined.
Typically, hydrogen gas for the fuel cell system is stored at high pressure in a gas storage system including one or more interconnected pressure vessels to provide the hydrogen gas fuel necessary for the fuel cell stack. The hydrogen gas storage system typically includes at least one pressure regulator as part of the various and numerous valves, gauges, and fittings necessary for operation of the hydrogen storage system, where the pressure regulator reduces the pressure of the hydrogen gas from the high pressure in the vessels to a constant pressure suitable for the fuel cell stack.
In one known type of fuel cell system, the hydrogen gas is injected from the gas storage system into the anode side of the fuel cell stack by an injector. The injector is controlled to maintain a desired pressure within the anode sub-system by regulating the injector flow to match the hydrogen being consumed. Typically this is done with a pulse width modulation (PWM) control signal where a duty cycle and a frequency are defined and a controller produces a control signal for the desired pulse. The pressure regulator may reduce the pressure of the hydrogen gas from a tank pressure of up to 875 Mpa down to approximately 800 kpa to provide a constant supply pressure to the injector. The injector then provides a pulsed flow to regulate the stack anode pressure in a range between 100 and 300 kpa. In maintaining the anode pressure, the hydrogen flow needed to power the fuel cell system is satisfied. It is important to note that both the regulator and the injector are needed to maintain an accurate pressure control over the full range of power transients for vehicle operation. The injector frequency and pulse width are controlled by feedback from an anode pressure sensor. In addition, the injector when open, may provide a high velocity flow to an ejector that recycles gas flow from the stack outlet to the stack inlet. This pulsed operation in conjunction with the recycled flow is crucial to maintain durable and stable system operation.
Over the life of a vehicle, an injector will undergo hundreds of millions of cycles of operation. Because of the high pressure of the hydrogen gas provided to the injector, as well as high temperatures associated with the fuel cell stack operation and internal injector friction, it is relatively common for a hydrogen fuel injector to have periodic failures. If the injector fails and does not provide the desired amount of hydrogen to the stack, anode starvation could occur, which could cause permanent damage to the cell electrodes. Further, the anode injector pulse train is modulated to manage both nitrogen and water management in the anode, which can also cause anode starvation.
The above described technique for monitoring whether the injector is operating properly using pressure feedback from the anode sub-system has generally been acceptable. However, the pressure sensor can also fail and/or not operate properly, which does not allow the pressure readings to determine injector failure. Models are incorporated into the system algorithm to operate the system in response to an anode pressure sensor failure for those operations that require the pressure reading. However, such models are not available to determine whether the injector has failed if the anode pressure sensor has also malfunctioned.